Manuel Rigo

High-precision U-Pb zircon age from the Triassic of Italy : Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs

The Triassic time scale is poorly constrained due to a paucity of high-precision radiometric ages. We present a 206 Pb/ 238 U age of 230.91 ± 0.33 Ma (error includes all known sources) for zircon from an ash bed in the upper Carnian (Upper Triassic) of southern Italy that requires a major revision of the Triassic time scale. For example, the Norian stage is lengthened to more than 20 m.y. The section containing the ash bed is correlated with other Tethyan sections and, indirectly, with the Newark astronomical polarity time scale (APTS). The dating provides also a minimum age for some important climatic and biotic events that occurred during the Carnian. We note a coincidence between these events and the eruption of the large igneous province of Wrangellia, but the possible link between volcanism and climatic and biotic events requires further scrutiny.

Palaeoecology of Late Triassic Conodonts: Constraints from Oxygen Isotopes in Biogenic Apatite

The oxygen isotopic composition of conodont apatite derived from the Late Triassic (Carnian to lower Norian), Pignola 2 and Sasso di Castalda sections in the Lagonegro Basin (Southern Apennines, Italy) was studied in order to constrain the habitat of Late Triassic conodont animals. Oxygen isotope ratios of conodonts range from 18.5 to 20.8‰ V-SMOW, which translate to palaeotemperatures ranging from 22 to 31°C, assuming a &dgr;18O of Triassic subtropical sea water of -0.12‰ V-SMOW. These warm temperatures, which are well comparable to those of modern subtropical-tropical oceans, along with the body features of the conodont animal suggest that conodont &dgr;18O values reflect surface water temperatures, that the studied conodont taxa lived in near-surface waters, and that &dgr;18O values of Late Triassic conodonts can be used for palaeoclimatic reconstructions.

Magnetostratigraphy, biostratigraphy, and chemostratigraphy of the Pignola-Abriola section: New constraints for the Norian-Rhaetian boundary

A detailed magnetostratigraphic investigation of the Pignola-Abriola section of Norian to Rhaetian age permits the identification of 22 magnetic polarity reversals grouped in 10 magnetozones. We correlate the magnetostratigraphy of the Pignola-Abriola section with the Newark astrochronological polarity time scale (APTS). In total, 19 correlation options were tested, and only one (option 7) yielded a statistically significant correlation that was consistent with the available information on the stratigraphic age of the Newark APTS. After some adjustments to minimize erratic variations in sediment accumulation rates, a final correlation (option 7.1) was used to generate an age model of sedimentation for the Pignola-Abriola section. The Pignola-Abriola section has been correlated with Rhaetian sections from the literature, notably the current global boundary stratotype section and point candidate for the base of the Rhaetian at Steinbergkogel, Austria, where the Norian-Rhaetian boundary is proposed to be placed at a stratigraphic level containing the first appearance datum (FAD) of conodont Misikella posthernsteini , traced on the Newark APTS to ca. 209–210 Ma. Issues regarding the taxonomy of M. posthernsteini , a species characterized by transitional forms with its ancestor Misikella hernsteini , lead us to propose the alternative option of placing the Norian-Rhaetian boundary at a prominent negative δ 13 C org spike observed in the Pignola-Abriola section at meter 44.5, 50 cm below the level containing the FAD of M. posthernsteini sensu stricto and close to the base of radiolarian Proparvicingula moniliformis zone. This level has been magnetostratigraphically correlated to Newark magnetozone E20r.2r at ca. 205.7 Ma. Assuming an age of ca. 201.3 Ma for the Triassic-Jurassic boundary, the Rhaetian Stage would have a duration of ~4.4 m.y.

Oxygen isotopic evidence for Late Triassic monsoonal upwelling in the northwestern Tethys

The oxygen isotopic composition of conodonts from the Pizzo Mondello section in the Sicani Basin (Sicily, Italy), on the northwestern Tethys margin, has been used to reconstruct regional paleoseawater conditions. The δ 18 O phos values (21‰–21.9‰) measured by sensitive high-resolution ion microprobe (SHRIMP II) are consistent with previously published δ 18 O phos data from the contemporaneous marginal Hallstatt-Meliata Basin (20.3‰–22.5‰) located farther north, but are distinctly higher than those previously reported from the open ocean setting of the nearby Lagonegro Basin (18.7‰–20.0‰). This ∼2‰ offset represents a temperature differential of ∼8 °C, suggesting that sea-surface temperatures along the northwestern margin (Sicani and Hallstatt-Meliata basins) of the Tethys Ocean were lower than in the open ocean (Lagonegro Basin) during the late Carnian (Tuvalian 2) to early Norian (Lacian). We interpret the cooler waters of the marginal basins to reflect coastal upwelling driven by strong monsoonal systems, as predicted by existing paleoclimate models for this region.

A new Metapolygnathus platform conodont species and its implications for Upper Carnian global correlations

A rich conodont fauna from two Neotethyan sections, Pizzo Mondello (western Sicily, Italy) and Pignola 2 sections (southern Apennines, Italy) includes conodonts described herein as a new species. Metapolygnathus praecommunisti sp. nov. is transitional between Paragondolella noah and Metapolygnathus communisti. The genus Metapolygnathus (including M. praecommunisti) is now characterised by posterior prolongation of the keel termination, associated with a centrally located pit and with a weak ornamentation confined to the anterior part of the platform margins. The establishment of M. praecommunisti addresses the problems related to the origin and the peculiar, probably facies-controlled, distribution of its descendant species M. communisti. Since M. praecommunisti occurs in the entire Tethys and in North America, we propose the species as a good guide fossil for global correlations, characterised by a short temporal range limited to the uppermost Tuvalian (upper Carnian). The stratigraphic occurrence of the genus Metapolygnathus is restricted to the Tuvalian—Lacian (upper Carnian—lower Norian), excluding its presence in the Julian substage (lower Carnian).

Application of numerical cladistic analyses to the Carnian–Norian conodonts: a new approach for phylogenetic interpretations

The high intraspecific variability of conodont platform elements in the upper Carnian–lower Norian interval and the proliferation of numerous species in this relatively short time have generated many problems for the understanding of Late Triassic conodont phylogeny, systematics and taxonomy. Since Late Triassic natural assemblages are still unknown, we have applied cladistic methodologies to investigate the evolution of Carnian–Norian conodont platforms and to infer more precise phylogenetic relationships among taxa. Numerical cladistic analysis was undertaken of species belonging to the five Late Triassic genera Paragondolella, Carnepigondolella, Metapolygnathus, Epigondolella and Norigondolella. A taxon–character data matrix describing the distribution of 64 characters amongst two outgroups and 31 ingroup taxa was compiled and processed using PAUP* 4.1. Our analyses show the evolutionary and systematic value of certain morphological characters, and lead to a reinterpretation of the phylogenetic position of the genera considered: Metapolygnathus, Epigondolella and Norigondolella are monphyletic taxa, Paragondolella represents a polyphyletic assemblage of basal members of the ingroup, and Carnepigondolella a paraphyletic group including primitive forms of the Metapolygnathus and Epigondolella lineages. Our analyses also clarify the systematic position of ‘Metapolygnathus communisti B’ and confirm the existence of four new species: Carnepigondolella angulata sp. nov., Epigondolella heinzi sp. nov., Epigondolella miettoi sp. nov. and Norigondolella trinacriae sp. nov.

A vertebrate nesting site in northeastern Italy reveals unexpectedly complex behavior for late Carnian reptiles

Abstract We interpret 13 large subcircular or horseshoe-shaped depressions discovered in Late Triassic peritidal carbonate rocks of the Dogna Valley in Udine Province, northeastern Italy, to be reptile nests. These trace fossils show truncation of strata, elevated ridges of massive sediment, and sediment infill within the depression differing in shape and sedimentary structures from the host sediment. The palynological assemblage of a shaly interbed close to the nest layer indicates a Tuvalian age (late Carnian). Archosaurian footprints, produced possibly by aetosaurs, are on a surface 130 cm above the nest-bearing layer. The trackmakers are considered the most probable nest makers.

The Norian-Rhaetian interval in two sections of the Lagonegro area: the transition from carbonate to siliceous deposition

New biostratigraphic data from two stratigraphic sections (Mt. St. Enoc and Mt. Volturino), of the Lagonegro Basin (Southern Apennines, Italy), define the age of the sedimentary events and delineate the sedimentary evolution for the Norian-Rhaetian time interval.The upper part of the Calcari con Selce Fm in the Mt. St. Enoc section is characterized by an alternation of cherty limestones rich in organic matter and thin silicified calcarenites with black cherty layers. The upper part of the Calcari con Selce Fm of the Mt. Volturino section exhibits instead intermediate characters between the Calcari con Selce and Scisti Silicei Fms, with cherty limestones, shales and radiolaritic beds. The biostratigraphic data based on conodonts and radiolarians allowed a good correlation among the Norian-Rhaetian successions throughout the Lagonegro Basin the two sections represent two different environments of the Lagonegro Basin. In fact, radiolarites (Scisti Silicei Fm) are present from the Misikella hernsteini-Parvigondolella andrusovi conodont Zone (late Sevatian) in the Mt. Volturino section while they appeared after the Misikella ultima conodont Zone (uppermost Rhaetian) in the Mt. St. Enoc section. This proves that euxinic carbonate facies and radiolaritic facies were deposited in the same time interval and in the same basin. On the basis of sedimentological analysis and with the support of biostratigraphy, the non-coeval beginning of biosiliceous sedimentation is discussed in order to point out the control factors: different paleobatimetry, upwelling areas or volcanic activity. Our preliminary results show that the Lagonegro Basin during the Norian-Rhaetian interval was probably characterized by an articulated physiography and that volcanism should have had an important role in changing pelagic sedimentation.

The late Norian-Hettangian stratigraphic and paleogeographic evolution of the Bergamasc Alps

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New Upper Triassic Conodont Biozonation of the Tethyan Realm

Conodonts are biostratigraphically very important microfossils in the Upper Triassic, occurring in different marine habitats, from deep-ocean to shallow-shelf waters. Because of their great abundance, worldwide distribution, strong resistance to rock metamorphism, and mineralogical composition that makes them reliable tools for biostratigraphic and geochemical studies, conodonts have proven to be important tools in defining the Geological Time Scale (GTS) and Global Stratotype Section and Points (GSSPs). We present here an original Upper Triassic conodont biozonation for the Tethyan Realm integrated, where possible, with ammonoid and radiolarian zones, providing also numerical ages for stages and substages. Based on the most recent conodont biostratigraphic and systematic studies, we propose a subdivision of the Upper Triassic interval into 22 conodont zones (nine for the Carnian, ten for the Norian, and three for the Rhaetian), correlated, where possible, with the most recent North American conodont zonations. Discussions on the most biostratigraphically important conodont taxa are also provided, in particular for the stratigraphic intervals around the base of the Norian and Rhaetian stages, the GSSPs of which have yet to be defined. In this view, we provide data supporting the validity of conodonts as reliable tools for global correlations, recommending two conodont biovents as possible primary biomarkers: the FAD (First Appearance Datum) of Metapolygnathus parvus for the base of the Norian and the FAD of Misikella posthernsteini for the base of the Rhaetian. The conodont species Norigondolella carlae n. sp. from the upper Tuvalian (Carnian) is also defined.